Role of martensite decomposition in tensile properties of selective laser melted Ti-6Al-4V

Highlights: • Degree of martensite decomposition was investigated in post-SLM annealed Ti-6Al-4V. • Retained $\left\{10\overline{1}1\right\}<10\overline{1}2>$ type twin was observed in 2 h annealed condition. • Retained martensite contained twins and dislocations reduced the ductility. • An equilibrium microstructure provided a strong and ductile SLMed Ti-6Al-4V. The as-Selective Laser Melted (as-SLMed) Ti-6Al-4V generally has a martensitic microstructure. In the present study, martensite decomposition was investigated in various annealed samples by XRD, SEM and TEM. It has been found that annealing at 700 °C or 800 °C for 2 h was not sufficient to obtain a fully decomposed microstructure. The amount of martensite did reduce greatly and volume fraction of β phase increased significantly when the annealing temperature increased from 700 to 800 °C. However, the best combination of mechanical property was not obtained. It is to be noted that annealing at 800 °C for 2 h is widely used for SLMed Ti-6Al-4V as reported in literature. Detailed SEM and TEM analysis revealed the presence of residual martensite containing twins and a high density of dislocations even in the 800 °C/2 h annealed sample. The twin was determined as $\left\{10\overline{1}1\right\}$ $10\overline{1}2$> type, which are retained twin formed in martensitic transformation during the cooling in SLM. Annealing at 800 °C for 6 h or more provided a fine equilibrium α + β microstructure, which provided a strong and ductile SLMed Ti-6Al-4V.